1. Field of the Invention
This invention relates to packet switched networks, and more particularly to a method of clock recovery in cell-relay networks, particularly ATM (Asynchronous Transfer Mode) networks offering constant bit rate services.
2. Brief Description of the Prior Art
AAL1 is the ATM Adaptation Layer for Constant Bit Rate (CBR) services. CBR services carry time sensitive data, such as voice and video. The original clocked (isochronous) data is segmented into ATM cells at the transmitting end, carried over the ATM network, and reassembled into a continuous clocked stream at the receiving end. The receiving end needs to provide a clock for this output data stream, or recover the transmitting end clock.
Various standardized and un-standardized clocking schemes are current used in ATM:                1. Line Rate Clocking: A clock is provided at the receiving end by an incoming data link with the same nominal clock rate as the transmitting end. This incoming clock will often need to be de-jittered before being used, because requirements on outgoing telecom clocks are usually more stringent than requirements on incoming telecom clocks.        2. Network Synchronous Clocking: A telecom clock is recovered from an 8 kHz reference clock. This is preferably a clock which is PRS-traceable (traceable to a Primary Reference Source). Standards for SDT (Structured Data Transfer) AAL1 require that a clock be recovered from the ATM Network clock, which is a multiple of 8 kHz. The 8 kHz must be frequency-multiplied up to the desired rate.        3. Adaptive Clocking: The clock from is recovered from information about the fill level of the receive buffer containing the received segmented data. This buffer serves to even-out the variation of the incoming data versus the receiver-end clock which is reading out the data. This method is referred to in the AAL1 standards as “Adaptive” but the exact method used is not subject to standardization.        4. SRTS Clocking: The clock is recovered from time stamps carried by the ATM cells. The method standardized for AAL1 is referred to as Synchronous Residual Time Stamps (SRTS) and described in U.S. Pat. No. 5,260,978.        
In the prior art, external PLL(s) as employed and an algorithm resident in CPU was used adaptive clocking and SRTS.
An object of the present invention is to provide a way of efficiently dealing with all four of these required clocking schemes in one circuit implementation.